Triangular waves are frequency signals that repetitively repeat the voltage change of a linear rise and fall and are typically used in pulse width modulation (PWM). For example, in PWM type feedback control, triangular waves and a feedback signal (PWM modulation input) are respectively input into a pair of input terminals of a comparator comprising an operational amplifier, and the voltage levels of two input signals are compared. In a section where the level of the triangular waves is higher than the feedback signal, a pulse string signal with a modulation pulse width as a first logic level (for example, H level) is generated, and in a section where the level of the triangular waves is lower than the level of the feedback signal, a pulse string signal with a modulation pulse width as a second logic level (for example, L level). Based on the pulse string signal, a switching element is switching-controlled.
When PWM control is simultaneously carried out by several subsystems in one system, several triangular waves are used. In this case, the application can be functional if the phases of these several triangular waves are appropriately shifted instead of being the same. For example, in an integrated circuit (IC) on which a multichannel switching regulator is mounted, a multiphase system that carries out a switching operation for several switching regulators on the same chip by shifting the phases is adopted to disperse the power consumption and to reduce the noise of an input line, and multiphased triangular waves are required.
FIG. 7 shows the configuration of a conventional multiphased triangular wave oscillating circuit used in PWM control. The multiphased triangular wave oscillating circuit is a two-phase type and has reference triangular wave oscillating circuit 100, differential amplifier 102, and output stage circuits 104 and 106.
The reference triangular wave oscillating circuit 100 comprises a radial wave oscillating circuit and an integrated circuit, for instance, and oscillates and outputs a reference triangular wave S with a fixed frequency close to nearly ideal waves. Reference voltages VRH and VRL corresponding to an upper limit wave crest value and a lower limit wave crest value of the reference triangular wave S are supplied to the reference triangular wave oscillating circuit 100 from a power supply circuit.
The differential amplifier 102 comprises a pair of differentially connected NMOS transistors 108 and 110, a constant current source 112 commonly connected to the sources of the two transistors, load resistors 114 and 116 respectively connected in parallel between the drains of the two transistors and a power supply voltage terminal VDD, and time constant capacitors 118 and 120, and is constituted as a voltage follower at a low slew rate. The reference triangular wave S from the reference triangular wave oscillating circuit 100 is input into the gate terminal of one NMOS transistor 108, and a reference voltage VRN equal to the central level of the reference triangular wave S is input into the gate of the other NMOS transistor 110. In the drain of the NMOS transistor 108, a triangular wave A with almost the same phase or a regular phase slightly delayed from the phase of the reference triangular wave S is obtained, and in the drain of the NMOS transistor 110, a triangular wave B with a phase opposite the phase of the regular-phase triangular wave A is obtained. The output stage circuits 104 and 106 comprise buffer circuits or driving circuits that drive and output two phased triangular waves A and B from the differential amplifier 102 toward each corresponding PWM comparator (not shown in the figure).
FIG. 8 shows an ideal relationship between the reference triangular wave S and the two phased triangular waves A and B, and an ideal relationship between the two phased triangular waves A and B. In the figure, there are several offsets δV between the upper limit wave crest value VRH and the lower limit wave crest value VRL of the reference triangular wave S, and the upper limit wave crest value VGH and the lower limit wave crest value VGL of the multiphased triangular waves A and B. The offset δV is due to the threshold voltage of the NMOS transistors 108 and 110.
As mentioned above, in the conventional multiphased triangular wave oscillating circuit, the reference triangular wave S is generated by the reference triangular wave oscillating circuit 100, the triangular wave A with a regular phase and the triangular wave B with an opposite phase are simultaneously generated from the reference triangular wave S by the differential amplifier 102, and the two triangular waves A and B are driven and output by the output stage circuits 104 and 106. The two phased triangular waves A and B are oscillated and output by a so-called open loop type. In this open loop type, the precision of the multiphased triangular wave depends largely on scattering in the characteristics of the circuit elements.
In particular, in the two-phase triangular wave oscillating circuit of FIG. 7, the element characteristics are set identically between identical left and right functional elements in the differential amplifier 102, that is, between the two NMOS transistors 108 and 110, the two resistors 114 and 116, and the two capacitors 118 and 120. However, if scattering exists in the element characteristics between identical functional elements, the balance of the waveform and the phase of the two phased triangular waves A and B collapses. Also, even if the characteristics are the same for the same functional elements, if they deviate from the original specified characteristics or if there is an error in the electric characteristics due to a relationship with respect to other functional elements, the precision of the triangular waveform is lowered.
Actually, in the two-phase triangular wave oscillating circuit of FIG. 7, for example, as shown in FIGS. 9A and 9B, a large scattering is seen in the waveform precision, phase relationship, waveform level, etc., of multiphased triangular waves A and B among mass-produced products. The case of FIG. 9A is entirely close to an ideal value (FIG. 8), however the peaks of two triangular waves A and B are blunted. In the case of FIG. 9B, the peaks of the two triangular waves A and B are not only further blunted, but the wave crest value deviates considerably from the ideal value (FIG. 8).
Thus, if the precision of the triangular wave supplied to each PWM comparator from the multiphased triangular wave oscillating circuit is not good, clearly, the PWM modulation effect is affected. For example, as mentioned above, if the peaks of an actual triangular wave A (similarly, triangular wave B) are blunted, as shown in FIG. 10, the actual PWM modulation effect (PWM comparator output) also deviates from an ideal value, resulting in a decrease in the precision of PWM control.
Also, in a PWM comparator, a voltage clamp (or limiter) function is installed so that the voltage level of the PWM modulation input cannot deviate by too large a level after shaking off (exceeding) the wave crest value of the triangular wave. In this case, the clamp level is set based on the ideal wave crest value; however if there is an error in the wave crest value of the actual triangular wave, the clamp function is also disturbed. For example, if the actual wave crest value deviates by a small absolute value when it separates from the clamp level, the clamp timing is delayed, so that a problem results in PWM control. For example, ripples are generated in the output of the switching regulator. Also, if the actual wave crest value deviates by a large absolute value when it exceeds the clamp level, the PWM modulation input is clamped in the wave crest value, so that a desired PWM modulation result cannot be obtained in the vicinity of the wave crest value (the vicinity of 0% duty or the vicinity of 100% duty).
In a general system for the PWM control, if a fine change of the PWM modulation input is Δe and a fine change of the PWM modulation result is Δm, the signal processing of the PWM modulation is regarded as having an amplification stage of Δm/Δe. Therefore, if the wave crest value of the triangular wave or the peak to peak value deviates from an ideal value or set value or the waveform (especially, the peak) is blunted, the small signal characteristic of the PWM modulation is not exerted as desired. As a result, the gain is too high, so that abnormal oscillation is likely, or the gain is too low, so that insufficient precision is likely.